The function of a conventional hearing aid is to amplify acoustic signals to make sounds audible to hearing-impaired individuals. Its basic structure consists of a microphone, an amplifier, a receiver and a power supply. The amplifier is the major component that magnifies the input speech signal. In the past five years, digital signal processing (DSP) has been introduced into hearing aid design. After analog speech signals are converted into digital form by an analog-to-digital converter, the signals can be manipulated by sophisticated processing algorithms before being converted back into the analog domain. Compared to standard analog hearing aids, digital aids provide more and precise controls over a broad range of parameters: the gain, frequency response and compression. Moreover, these settings can be individually programmed in each frequency band. Current digital hearing aids allow much detailed controls over hearing aid functions, but its one and only function is to amplify the signal.
Two types of amplification are used in hearing aid design. The linear amplifier limits the maximum output from peak clipping, which occurs when the electrical signal exceeds the maximum output of some component of the hearing aid circuit or when the digital signal exceeds the maximum digital number a finite number of bits can represent. This limitation causes various forms of distortion that reduces the intelligibility and subjective quality of speech. Current hearing aids use a non-linear amplifier, which reduces the gain as the output or input approach the maximum values. Compression is implemented by an analog circuit or by a digital processing algorithm to reduce the gain of the instrument when either the input or output exceeds a predetermined level. This type of amplification results in a wider dynamic range input to hearing-impaired patients, making soft sounds audible without making loud sounds uncomfortably loud. However, amplitude compression also changes the temporal properties of the original speech signal and may cause side effects in speech intelligibility. We will extend this point in our research.
Conventional hearing aids do not work for all hearing impairments. The primary function of conventional hearing aids is to amplify and make the speech signal audible within the constraints of a person's hearing thresholds and loudness tolerance levels. They solve the problem of hearing loss only when it is the amplification function of the ear that is defective, such as in sensorineural hearing loss due to outer hair cell loss and/or damage. No matter how sophisticated the instrument is, this type of hearing aid cannot solve the problem for other types of hearing loss, such as neural fiber removal in tumor-treated operations, which leave patients with little or no residual hearing, damage in inner hair cells, neuropath or brainstem, which not only affect intensity discrimination but also introduce sound distortion.
Digital signal processing allows for more complicated algorithms that may be used to compensate for these types of hearing loss. The transposer hearing aid is one such example designed to help patients without residual hearing at high frequencies. High frequency speech sounds are transposed and delivered to the low frequency region where patients are likely to have more residual hearing and more likely to be able to use that information. In this transposition process, high-frequency consonants are squeezed and transposed to the low-frequency range with original low-frequency vowels and consonants untouched. Although the original input is distorted and an unnatural sound is produced, more useful information is delivered to the audible frequency range, improving the user's perceptual capacity.
Neither conventional nor transposer hearing aids have achieved much success on patients with auditory neuropathy, a recently discovered hearing disorder that has unique pathologies and perceptual consequences. Auditory neuropathy may involve loss of inner hair cells (IHC), dysfunction of the IHC-nerve synapses, neural demyelination, axonal loss or possible combinations of any of the above. Clinically, these pathologies may be mixed with traditional cochlear impairment involving OHCs and/or central processing disorders involving the brainstem and cortex. Because one possible neural mechanism underlying the AN symptoms is the desynchronized discharge in the auditory nerve fibers, auditory neuropathy has also been termed “auditory dys-synchrony.” Auditory neuropathy not only causes sound attenuation, but also sound distortion, which cannot be compensated by either conventional or transposer hearing aids. New processing strategies should be developed to rectify the problem of sound distortion.
Clinical and psychoacoustic testing on auditory neuropathy subjects have been conducted to investigate the root causes of sound distortion. Pure-tone audiograms of auditory neuropathy subjects show a global trend opposite to regular hearing impairment—high thresholds at low frequencies but low or relative normal thresholds at high frequencies—implying that amplifying energy at high frequencies or transposing high-frequency components to the low-frequency range may not help. Test results from the temporal modulation transformation function (TMTF) show that auditory neuropathy patients have poorer temporal modulation discrimination ability than normal-hearing and other hearing-impaired people. It again implies that conventional hearing aids will not work for them since their degraded temporal modulation cannot be compensated. In addition, data from gap detection tests showed lower gap discrimination ability in auditory neuropathy than other hearing impairments, suggesting that auditory neuropathy patients have impaired temporal processing ability, which cannot be compensated by the conventional and transposer hearing aids. New strategies may be developed based on these clinical and psychoacoustic data to solve the problem of sound distortion in auditory neuropathy.
Various strategies have been proposed to help auditory neuropathy patients to hear clearer. One strategy is to increase modulation index in each different frequency band to compensate for the temporal modulation loss due to desynchronized discharges in the auditory nerve fibers in auditory neuropathy. This can be implemented over each extracted envelope in each frequency band and implemented by directly increasing the amplitude of peaks and decreasing the amplitude of troughs in a local temporal range. This method is definitely different from the amplification process used in conventional hearing aids, which amplify both the peaks and troughs. The conventional hearing aids keep the modulation depth the same as the original signal in linear compression, or even decrease the modulation depth in nonlinear compression. The amplitude of peaks cannot be amplified by the same ratio as the amplitude of valleys in nonlinear compression and worsened performance is predicted because of the degraded temporal modulations introduced in conventional hearing aids. The proposed strategy will change the amplitude of peaks and troughs in the opposite direction increase the fluctuations in temporal envelope in each frequency band. Most previous studies testified the importance of the amplitude modulation in speech intelligibility, but enhancement of the modulation has not been used in hearing aid technology and auditory neuropathy, to the best of our knowledge.
Aside from compensating for the temporal amplitude modulation deficit, the new strategies also compensate for hearing loss at low frequencies in auditory neuropathy. One strategy is to filter out all low frequency components based on psychoacoustic observations that auditory neuropathy patients have extremely poor pitch perception at low frequencies but relatively normal pitch processing at high frequencies. The high-pass filter's cutoff frequency is set based on the individual's audiogram. The assumption is that the distorted low frequency processing may confound auditory neuropathy patients' pitch perception at high frequencies. Once the part of signal that causes sound distortion is removed, higher speech recognition performance should be achieved.
Another strategy has been to compensate for the low frequency hearing loss by transposing low frequency components to high frequency range based on the individual's audiogram. We note that this frequency transposition is in the opposite direction as implemented in current transposing hearing aids, which typically transpose high-frequency signals to the low-frequency region to solve the lack-of-audibility problem at high frequencies. Both frequency components in low frequency range, in which no signal is audible even after being maximally amplified, and frequency components in the audible higher frequency range will be linearly or nonlinearly shifted to the higher frequency range. This processing shifts all frequency components, including the original audible high frequency components, which may make the processed sound have unnatural voice quality.